I'll almost always lean on separate packages for any shared logic like that (at least if I can use the same language on both ends).

For JS/TS, I'll have a shared models package that just defines the schemas and types for any requests and responses that both the backend and frontend are concerned with. I can also define migrations there if model migrations are needed for persistence or caching layers.

It takes a bit more effort, but I find it nicer to own the setup myself and know exactly how it works rather than trusting a tool to wire all that up for me, usually in some kind of build step or transpiration.